For years, those skilled in the art have made continuous efforts to develop pressure sensors that are low in cost and capable of being mass produced, while exhibiting high reliability, sensitivity and linearity. Certain conventionally known pressure sensors have been known to include semiconductor materials with a micromachined sensing diaphragm. In the processing of such structures, a thin diaphragm is often formed in a silicon wafer through chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming piezoresistive elements whose electrical conductivity changes with strain, such that deflection or deformation of the diaphragm causes a change in resistance of the piezoresistive elements. These changes correspond to the magnitude of pressure applied to the diaphragm. However, silicon is susceptible to chemical attack and erosion, particularly in environments such as where a high-pressure medium is to be sensed. For example, automotive applications that can involve sensing pressures of brake fluid, oil, coolant, transmission fluid, hydraulic fluid, fuel, steering fluid, and engine cylinders, at pressures of two atmospheres or more. For such applications, a pressure sensor must be physically resilient, and resistant to the hostile environment of the sensed medium.
As an alternative to silicon, diaphragms can be formed from metals or ceramics, which tend to be more physically resilient, and resistant to the hostile environment of a sensed medium as compared to silicon. One pressure sensor design positions a sensing component onto a base component comprising a diaphragm assembly that includes a diaphragm at its center, wherein the diaphragm comprises a metal or ceramic. On top of the diaphragm is a dielectric layer. On top of the dielectric layer is generally a plurality of piezoresistive elements.
Although commercially available dielectric compositions, such as DuPont QM44H™, DuPont 5704™, DuPont QS4200™ and ESL 9505-C™, may have good CTE and chemical compatibility with respect to certain ceramic substrates (e.g. alumina), such dielectric compositions generally do not provide have good CTE compatibility and in some cases chemical incompatibility with metal or metal alloy substrates, such as steels. Significantly, CTE mismatches between the substrate material comprising the diaphragm and the dielectric layer on top can result in significant stress caused by differential expansion which can result in bowing and distortion of the diaphragm which can render data from the pressure sensor to be erroneous, or in extreme cases the pressure sensor even becoming inoperable. Bowing and distortion is generally most prevalent in high temperature applications and is also known to occur during fabrication of the pressure sensor itself, particularly during the cool-down cycle associated with the high temperature dielectric firing step (e.g. 850° C.).